What is a precipitation reaction in a phase diagramm?
What is a eutectoidic reaction in a phase diagramm?
What is the driving Force between phase changes?
The decrease in net free Enthalphy
At which concentration of C is the border between steel and cas iron?
2.11 % weight percent
What are the Products Austenite can transform into and what parameter decides it?
Cooling rate lowest to highest: Ferrite, Bainite, Martensite
What is the influence of the grain size on the precipitation of ferrite from austenite?
Fine grain sizes cause a supersaturation of C in the inner Autenite grain, which makes nucleation more difficult.
Big grain sizes lead to a nucleation at less favourable points due to the carbon needing more time to diffuse into the center.
What is the most important nucleation site for ferrite precipitation?
Grain boundary or intergranular precipitation
How does a increase of the supercooling effect the content of ferrite?
The increase in cooling rate or supercooling decreases the amount of ferrite.
Name the equation that is used for calculating the transformed fraction of a phase to another at a Temperature dependant on the time.
Equation of Johnsson - Mehl-Avrami
Give the Johnsson - Mehl-Avrami equation
What is the Name of the eutectoidic component in the Fe-C diagramm?
Pearlite
What is the relation between intergranular spacing and hardness in Pearlite?
By decreasing intergranular spacing hardness increases.
Describe the concentration of Cementite for Pearlite forming.
In which ways can pearlite grow?
Nucleation of new lamellae
Branching of new lamellae
Extension of the terminal part of the lamellae
How can the dependancy between diffusion coefficient and temperature be described?
E=Q=energy barrier/activation energy
R=universal gas constant
T=temperature
D_0=preexponential factor
What is a hyper or hypo-eutectic alloy?
A higher (hyper) or lower (hypo) deviation from the concentration of the eutectic
Can an alloy containing 0.6% of C be transformed into a pearlitic microstructure? Why?
Yes, because the undercooling can supersaturate the Austenite and enable the Cementite precipitation at a high enough speed.
What are influencing elements for the Pearlite transformation Temperature?
lowering the temperature: Ni,Mn
raising the temperature: Si,Cr,Mo
What are heat treatments to achieve a ferritic perlitic microstructure?
full anealing with furnace cooling
Normalizing with air cooling
What are the effects of Pearlite content on toughness?
in steels %C>0.3 a finer pearlite shows higher strength
in steels %C<0.3 the pearlite content does not affect the yield strength, but tensile strength increases due to higher work hardenability
on plain C steels pearlite has a decreasing toughness effect
What is Bainite?
Bainite is a second eutectic constituent composed of ferrite and cementite.
It forms in two steps and the formation mechanism differs from the formation of pearlite:
Formation of acicular ferrite
cementite appears and remaining austenite transforms into mixture of Ferrite and Cementite
How are lower and upper Bainite distinguished?
Upper Bainite: high forming T
lower Bainite: low forming T
How can the nose of Bainite forming be seperate from the perlitic nose?
By alloying Carbid forming Elements.
What is the analogy between Martensitic and Bainitic transformation
There is a Bainit start and finishing temperature.
The Bainite start temperature enables the Bainite formation.
The Bainitic finish temperature determines the temperature at which when cooled to this temperature the whole structure will have converted to martensite.
which elements influence the Bainit start temperature?
With increase in these elements the Bainit starting temperature drops:
C,Mn,Ni,Cr,Mo
How can cementite formation in Bainite be avoided?
Sufficient alloying of Si and Al
What is the structure of upper Bainite?
Similar to Perlite. Ferrite laths (Leisten) with interlath cementite.
At high temperature grows in competition with perlite.
At low temperature size of laths so small, that can only viewes by TEM.
What is the structure of lower Bainite?
Ferrite plate formation at grain boundaries with very small Cementite precipitations, due to low diffusivity.
Very high driving force.
What is the difference between Bainite and Perlite?
diffusive froming of Perlite, non diffusive forming of Bainite
lamellar vs plate structure
Bainite: First ferrite forming, cementite after
Perlite: both form simultainiously
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